Add custom exception for fixIO

Traditionally, `fixIO f` throws `BlockedIndefinitelyOnMVar` if
`f` is strict. This is not particularly friendly, since the
`MVar` in question is just part of the way `fixIO` happens to be
implemented. Instead, throw a new `FixIOException` with a better
explanation of the problem.

Reviewers: austin, hvr, bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #14356

Differential Revision: https://phabricator.haskell.org/D4113

5 files changed, 19 insertions, 3 deletions

diff --git a/libraries/base/Control/Exception/Base.hs b/libraries/base/Control/Exception/Base.hs
index a15cc8ed32..d443159cb3 100644
--- a/libraries/base/Control/Exception/Base.hs
+++ b/libraries/base/Control/Exception/Base.hs
@@ -30,6 +30,7 @@ module Control.Exception.Base (
         NonTermination(..),
         NestedAtomically(..),
         BlockedIndefinitelyOnMVar(..),
+        FixIOException (..),
         BlockedIndefinitelyOnSTM(..),
         AllocationLimitExceeded(..),
         CompactionFailed(..),
diff --git a/libraries/base/GHC/IO/Exception.hs b/libraries/base/GHC/IO/Exception.hs
index 9203f46828..020bc067df 100644
--- a/libraries/base/GHC/IO/Exception.hs
+++ b/libraries/base/GHC/IO/Exception.hs
@@ -33,6 +33,7 @@ module GHC.IO.Exception (
 
   ArrayException(..),
   ExitCode(..),
+  FixIOException (..),
 
   ioException,
   ioError,
@@ -268,6 +269,15 @@ instance Show ArrayException where
         . (if not (null s) then showString ": " . showString s
                            else id)
 
+-- | @since TODO
+data FixIOException = FixIOException
+
+-- | @since TODO
+instance Exception FixIOException
+
+instance Show FixIOException where
+  showsPrec _ FixIOException = showString "cyclic evaluation in fixIO"
+
 -- -----------------------------------------------------------------------------
 -- The ExitCode type
 
diff --git a/libraries/base/System/IO.hs b/libraries/base/System/IO.hs
index fde5bb66e5..68817247d2 100644
--- a/libraries/base/System/IO.hs
+++ b/libraries/base/System/IO.hs
@@ -400,10 +400,15 @@ withBinaryFile name mode = bracket (openBinaryFile name mode) hClose
 -- ---------------------------------------------------------------------------
 -- fixIO
 
+-- | The implementation of 'mfix' for 'IO'. If the function passed
+-- to 'fixIO' inspects its argument, the resulting action will throw
+-- 'FixIOException'.
 fixIO :: (a -> IO a) -> IO a
 fixIO k = do
     m <- newEmptyMVar
-    ans <- unsafeDupableInterleaveIO (readMVar m)
+    ans <- unsafeDupableInterleaveIO
+             (readMVar m `catch` \BlockedIndefinitelyOnMVar ->
+                                    throwIO FixIOException)
     result <- k ans
     putMVar m result
     return result
diff --git a/testsuite/tests/mdo/should_fail/mdofail006.stderr b/testsuite/tests/mdo/should_fail/mdofail006.stderr
index ea186c0076..e2cf503df7 100644
--- a/testsuite/tests/mdo/should_fail/mdofail006.stderr
+++ b/testsuite/tests/mdo/should_fail/mdofail006.stderr
@@ -1 +1 @@
-mdofail006: thread blocked indefinitely in an MVar operation
+mdofail006: cyclic evaluation in fixIO
diff --git a/testsuite/tests/typecheck/should_compile/holes2.stderr b/testsuite/tests/typecheck/should_compile/holes2.stderr
index d7484fa142..fd3073d377 100644
--- a/testsuite/tests/typecheck/should_compile/holes2.stderr
+++ b/testsuite/tests/typecheck/should_compile/holes2.stderr
@@ -9,7 +9,7 @@ holes2.hs:3:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
         instance Show Ordering -- Defined in ‘GHC.Show’
         instance Show Integer -- Defined in ‘GHC.Show’
         ...plus 23 others
-        ...plus 61 instances involving out-of-scope types
+        ...plus 62 instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: show _
       In an equation for ‘f’: f = show _